Heterogeneous catalysis (C9981)
Common Pitfalls of Catalysis
Manuscripts Submitted to Chemistry
of Materials
Active site/catalyst evaluation
• Turn-over frequency (TOF; [s–1])
= number of catalytic cycles performed by 1 active
site per time unit
• Precise numbers for homogeneous and enzymatic
catalysis
• Numbers for heterogeneous catalysis???
TOF [s–1]
Hetero: ~1–100 s–1
Homo: ~10–1000 s–1
Enzymes: ~10000–1000000 s–1
Active site/catalyst evaluation
• Turn-over number (TON; [-])
= number of catalytic cycles performed by 1 active
site before deactivation (~lifetime)
• Precise numbers for homogeneous and enzymatic
catalysis
• Numbers for heterogeneous catalysis???
Common pitfalls - 1
• Improper calculation of turnover frequencies
(TOFs)
– Low conversion, early stage of the rxn
– Valid only for specific reactant concetration
• Improper calculation of turnover numbers
(TONs)
– A measure of catalyst’s stability
– Accurate determination requires measurements until the
catalyst’s activity is completely lost
Common pitfalls - 2
• Deactivation studies at full/equilibrium
conversion
– Batch vs. continuous flow
– The available amount of reagents limit the conversion =
the catalyst could be in fact more active
– Deactivation should be studied at intermediate
conversions
Common pitfalls - 2
• Deactivation studies at full/equilibrium
conversion
Active site/catalyst evaluation
• Selectivity is ability of catalyst to form one product
from a pool of products (possibly many)
• Selectivity (S; [%])
= number of D molecules produced / R molecules
converted
reactant R
Common pitfalls - 3
• Comparison of selectivities at different
conversion levels
– Selectivity does depend on conversion
– A→B→C
– Always compare at isoconversion
Diffusional limitation
• Gradient of reactant concentration in
– Fluid film of particle (External diffusion)
– Inside the pore (Internal diffusion)
Common pitfalls - 4
• Neglect of mass transfer (diffusional)
limitations
– External (film) + Internal (pore) diffusion
Rate determining step
• High temperature
– The chemical reaction is fast
– There is no time for internal diffusion to take place, only
external surface employed in catalysis
– Diffusional steps
are limiting
– Eapp = Ea of the diffusion
in the fluid film
(external diffusion)
Common pitfalls - 4
• Neglect of mass transfer (diffusional)
limitations
– Batch, both diffusions: stirring rates
– Continuous flow, external diffusion:
– Continuous flow, internal diffusion: particle size, pore
volume, pore size
Diffusional limitations
• Internal diffusional limitations always present to
some extent
– We can diminish them at the time of catalyst preparation
(pore volume, pore diameter, size of catalysts grains)
– Good practice is to compare a series of catalysts with
similar pore volume, pore diameter, and size of catalysts
grains
• External can be avoided
at the time of catalytic
reaction
– Linear velocity of vector gas,…
Common pitfalls - 5
• Failure to study the catalysts after reaction
– Catalysts can change dramatically during catalytic reactions
– Sintering, coking, pore collapse, poisoning,…
– It is not correct to justify the differences between the
catalysts only based on the characterization of the starting
material
Common pitfalls - 5
• Failure to study the catalysts after reaction
– Catalysts can change dramatically during catalytic reactions
– Coking
Common pitfalls - 5
• Failure to study the catalysts after reaction
– Catalysts can change dramatically during catalytic reactions
– Sintering
Before cata After cata
Common pitfalls - 6
• Failure to consider differences in surface area
of catalysts
– Catalytic activity scales with number of active sites
– Common sense: Number of active sites scales with the
surface area
– Compare the activity of catalysts per m2